(1) Field of the Invention
The present invention relates to a burner for burning liquid fuel used for a small installation for combustion such as a small boiler for business use, and more particularly relates to what is called a low NOx gas burner by which a small amount of nitrogen oxide (NOx) is generated.
(2) Related Art of the Invention
Recently, from the viewpoint of environmental preservation, it is strongly desired to reduce an amount of NOx gas generated by various combustion apparatus such as medium and large boilers and industrial furnaces, and also generated by small installations for combustion such as small boilers for business use. Therefore, it is urgent to reduce an amount of NOx gas generated by combustion apparatus.
Especially, in the case of a small installation for combustion such as a small boiler for business use in which liquid fuel is burnt, the load of a combustion chamber is heavy, so that the combustion condition is severe. As a result, an amount of NOx gas generated by the installation for combustion tends to be increased.
Conventionally, in order to reduce an amount of NOx gas generated by combustion apparatus, the following methods are employed: a recirculation method in which a portion of exhaust gas is returned to a combustion section and burnt again so as to lower the combustion temperature; a two-step combustion method in which fuel is burnt in two steps; and a method in which a normal burner is changed to what is called a low NOx gas burner by which a small amount of NOx gas is generated.
By this low NOx gas burner, the combustion temperature is lowered, so that the generation of thermal NOx gas can be suppressed. For example, the following low NOx gas burners are provided: a burner in which fuel injection ports are located distant from each other and flames are located at distant portions, so that the flame temperature can be lowered; a burner by which fuel is slowly burnt when combustion air is supplied by two steps; and a burner by which fuel is injected stepwise so that fuel-rich flames and fuel-lean flames are formed.
According to the above method of gas recirculation and that of two-step combustion, an amount of NOx gas can be effectively reduced. However, the above methods are disadvantageous in that the cost is increased when the apparatus for adopting the methods is modified or extended.
Further, in the case of a small installation for combustion, problems are caused to ensure a space where the apparatus is installed.
On the other hand, according to the above method in which the burner is changed to a low NOx gas burner, problems are not caused in the cost and space. Therefore, it is most preferable to employ the above method in which the burner is changed to a low NOx gas burner. However, in the case of a burner for burning liquid fuel, there is not provided a burner which can reduce an amount of NOx gas to a regulation value only by its own function. In almost all cases, it is necessary to provide not only a low NOx burner but also a secondary installation for reducing an amount of NOx gas, for example, an installation in which NOx gas is removed with ammonia, an installation in which the combustion temperature is lowered with a catalyst, and the like are required. In the case of a conventional low NOx gas burner, the aforementioned slow combustion or thick and thin fuel combustion is conducted. Therefore, the flame area is increased or the flame length is extended. Accordingly, it is difficult to apply the conventional low NOx gas burners to small boilers and other.
Due to the above problems, under the present conditions, satisfactory countermeasures are not taken so as to reduce an amount of NOx gas in a small installation for combustion such as a small boiler.